Exploratory studies are proposed to detect the effects of prolonged administration of narcotics (morphine and merperidine) on thalamic nociceptive activity in the laboratory rat. The animals will be maintained on the addiction schedule of Kerr and Pozuelo (Mayo Clin. Proc. 46:653-665, 1971) and at various intervals following pretreatment they will be used for acute electrophysiological experiments. They will be prepared for microelectrode recording of neural activity under chloralose-urethane anesthesia, but during the recording of nociceptive activity they will be maintained on local anesthesia. A digital computer will be used to identify nociceptive neurons by the unique spacing of action potentials they emit in response to pricking the skin of the animal's foot with a needle. Since it has been shown that small doses of morphine sulfate alter the unique spacing of action potentials in a predictable manner, this newly developed method can be used to detect subtle changes in nociception. Preliminary experiments have indicated that repeated administration of morphine sensitizes the nociceptive system at the thalamic level, therefore, experiments exploring the extended effects of morphine on thalamic nociception appear worthwhile. Furthermore, the effects of morphine will be compared to those of merperidine (Demerol R) not only at the onset of drug dependence but also during drug withdrawal.